In recent years, methods and apparatus for creating evoked potentials in the neural pathways of higher level organisms (e.g., animals and humans) have been developed. Evoked potentials can produce observable movements and/or analyzable electric signals (e.g., brain waves). Evoked potentials are created by stimulating neural pathways. In the past, three major types of stimulators have been used to create evoked potentials--light, sound and electrical stimulators. Light and sound stimulators have been used to stimulate the sensory and neural pathways associated with the eyes and ears. Electric stimulators have been used to stimulate motor neural pathways and the sensory neural pathways associated with somatic sensations, i.e., sensations associated with the sense of touch. The present invention is directed to electric stimulation.
In the past, the most common way to electrically stimulate motor and somato sensory neural pathways has been to attach a pair of spaced-apart electrodes to the body at the stimulus location. When an electric potential is applied to the electrodes, a current flow through the body is created. The current flow produces an electric field that disrupts the polarization of neurons located in the field (commonly called depolarization of the neurons) causing an evoked potential "message" to be transmitted along the neural pathway formed by depolarized and other neurons that define the neural pathway.
While electric stimulation using a pair of spaced-apart electrodes has certain advantages, it also has several disadvantages. In order to overcome certain of these disadvantages, in particular, the shallow penetration of electrode stimulators, magnetic stimulators have been developed. As described in U.S. Pat. application Ser. No. 008,210 entitled "Method and Apparatus for Magnetically Stimulating Neurons," filed Jan. 28, 1987, now U.S. Pat. No. 4,940,453, and assigned to the assignee of the present application, magnetic stimulators have the advantage of being noncontacting, relatively pain-free and capable of stimulating deep, normally inaccessible, nerves.
In the past, magnetic stimulators have stimulated neural pathways by energizing a pancake-shaped coil placed flat on the skin of a patient overlying the neural pathway to be stimulated. The energized coil creates a magnetic field that depolarized the neurons underlying the stimulation site. In comparison with electrode stimulators, one disadvantage of using pancake-shaped coils placed flat on the skin of a patient to stimulate neural pathways is a lack of stimulation site accuracy. More specifically, even though electrode stimulators have other disadvantages, when compared to magnetic stimulators, electrode stimulators are more accurate since the stimulating field produced by the electrodes is more confined than is the stimulating field produced by pancake-shaped magnetic coils placed flat on the skin of a patient. This invention is directed to magnetic stimulator coils that overcome this disadvantage.
One major disadvantage of magnetic stimulators is their large power requirements. More specifically, the relatively high voltage and current required by magnetic stimulators to produce a depolarizing magnetic field necessitate that magnetic stimulators have large power supplies. Part of the reason for the high voltage and current requirement is the lack of efficiency of the coils used in the past to create magnetic stimulation. This invention is also directed to providing magnetic stimulator coils that are more efficient than magnetic stimulator coils used in the past.